A few questions

My best guess. I modified the scale in Layout -> Page settings to make the staff and notes bigger. I also played with the stretch of measures (select a measure and Layout -> More/Less stretch) and I forced the line breaks. Also changed the size of the title frame.

Did you read the Handbook articles I cited? Pay particular attention to the description of the Page Fill Threshold - that is why you are having trouble controlling the distance between systems. It's worded a bit sketchily, so let me try to explain it more clearly. The Page Fill Threshold is a percentage, and if your page exceeds that percentage, then it is automatically stretched out to fill the page. So if you want to see the actual effect of your system distance settings, start by setting Page Fill Threshold to 100%, so the page is never artificially stretched out. Then, when you have the spacing about the way you want, you can reduce the Page Fill Threshold again to clean up the margins.

The staff size you have in the score you uploaded, though, is actually only barely shy of the industry standard staff size (measure some published charts to compare if you like). If you prefer unusually large print, that's OK - simply change the setting we already described: Layout->Page Settings->Space. But surely this isn't what you meant when you called it way too small to read on stage. So I'm guessing you already upped the size before posting? If it was smaller than that originally, then I am guessing maybe this is a part generated from a score, and that you (or the template you started from) had reduced staff size in order to fit all your instruments on a page. And then when you extracted parts, this smaller staff size was kept. If your score uses small staves, you need to increase the size in the parts. What I usually do is make a copy of the score and change the staff size (and paper orientation) *before* generating parts. 2.0 will have a facility to allow parts to automatically have different formatting than the score, although currently this doesn't apply to staff size.

What I would have done with your score is up the staff size from a Space setting of 1.753 (where you had it; industry standard is 1.764) to 2mm (slightly larger than standard; I like this for jazz charts, which are often sight-read). Simply doing that and nothing else fits nine staves per page, and your music would fit. Then it would just be a matter of adding some manual line breaks (click bar line, hit Enter) to fine tune where they occur.

Your system distance is set very low, but you aren't seeing it because the Page Fill Trheshold is kicking in. Set Page Fill Threshold to 100% and you'll see just how tiny you've actually set the system distance. The Page Fill is bailing you out in this case. If your part were longer, though, it would have tried to fit too many systems on the page (11). You'd be better off using a more reasonable system distance. Then, reduce the threshold again to clean up the bottom margins.